/Engine/Source/Runtime/GeometryCore/Public/PlaneTypes.h
The "positive side" of the plane is the half space to which the plane normal points. The "negative side" is the other half space. The function returns +1 when P is on the positive side, -1 when P is on the the negative side, or 0 when P is on the plane.
int WhichSide ( const [UE::Math::TVector](API\Runtime\Core\Math\TVector)< RealType > & P ) const